1. Field of the Invention
The present invention relates to a method for reducing compressed air losses when driving underground cavities which are supported by compressed air; the part of the inner cavity surface exposed by mining is sealed by a viscous medium, such as a bentonite suspension, after breaking away the earth.
When building tunnels or galleries in loose rock, the pneumatic driving method is used. In this case, an earth-working apparatus mines the face which is formed by the exposed end face of the underground cavity. The earth-working apparatus operates in a compressed air chamber, which is sealed off by a protective shield and an associated partition. The compressed air chamber forms the intermediate member between the solid rock or earth and the finished tunnel cavity. The compressed air in the chamber between the face and the shield partition prevents penetration of water into the working chamber. However, depending on the nature of the solid earth, part of the supporting compressed air escapes through the face. Therefore, in the case of permeable earth, this results in a high consumption of compressed air.
2. Description of Prior Art
With one known method, the consumption of compressed air is reduced by sealing the face by spraying on a viscous medium, which is preferably a bentonite suspension. The viscous medium is applied as a membrane which is only several centimeters thick, and seals the pores between the individual grains of earth from the compressed air. In this case, a drawback is that the face which is to be sealed is frequently loosened and disturbed or even destroyed by being sprayed with the viscous medium, so that the sealing membrane is not fully effective.
In another known method, the face is sealed against escaping compressed air in that, in front of the face, a chamber is provided that is bounded by bulkheads and is completely filled with a bentonite suspension, which is then acted upon by compressed air. This method for sealing the face is, however, very complicated and expensive.
An object of the present invention is to provide a method whereby the face can be provided with a fully effective sealing membrane in a simple and economical manner, and in an unerring manner.